The ultimate goal of this research is to contribute to our understanding of the neuronal machinery that is responsible for vasopressin release by the hypothalamic magnocellular neuroendocrine cells. We propose to study the response characteristics of the whole neuronal population by measuring plasma arginine vasopressin and of individual neurons by recording the activity of antidromically identified single supraoptic and paraventricular neurons in the unanesthetized monkey. We hope that these studies will further define the electrophysiological characteristics of hypothalamic vasopressinergic neuroendocrine cells and their role in the homeostasis of body fluids.